Fats are one of the very important component of our diet. But they are highly unstable toward atmospheric oxygen and start producing unpleasant smell. These undesirable compounds generated by degradation of fats are very harmful for our health. They are Carcinogenic in nature.

1. DATE: 06/03/2014

2. LIPID
OXIDATION
Causes & Prevention
Sadanand Patel

3. Presentation Pattern
 Rancidity(brief), Types
 Hydrolytic rancidity(brief)
 Oxidative rancidity and ways and outline
 Photo-oxidation(detail)
 Auto-oxidation(detail)
 Feasibility of reactions: Pro-oxidants
 Comparison b/w Photo and Auto-oxidation
 Hydroperoxides decomposition(brief)
 Factors affecting Lipoxidation(detail)
 Anti-oxidant, types and mechanism(detail)
 Measurement of Oxidative rancidity(brief)
 Fat oxidation characteristic curve and induction period
 General information
 Quick history of Lipoxidation

4. What is Rancidity?
Rancidity is a term generally used to denote a
condition of unpleasant odours and flavours in
foods resulting from deterioration in the fat or oil
portion of a food.
RANCIDITY
HYDROLYTIC
RANCIDITY
OXIDATIVE
RANCIDITY
MICROBIAL
RANCIDITY

5. Active sites
Oxidation
Hydrolysis
1
2

6. HYDROLYTIC RANCIDITY
Partial or Complete Hydrolysis
of ester bonds
Liberation of FFA
DAGs, MAGs,, Glycerol
EX:
Development of rancid flavour in milk
Deep fat frying
Enzyme action,
Heat, Moisture

7. OXIDATIVE RANCIDITY
Major causes of food spoilage
Extensive research have done, yet to be discovered
……extremely complex subject because
1. Sensitivity of
Intermediates
Isolation separation and
identification
2. Effect of catalyst
Pro & anti-oxidants
3. Influence of photo-
oxidation and auto-oxidation
simultaneously
4. Relation between
oxidation rate and stability

8. Photo-oxidation
Enzymatic oxidationAuto-oxidation
Ways
Fats & Oils
Primary
Oxidation
Products
Secondary
Oxidation
Products
Tertiary
Oxidation
Products
Factors Decomposition
Oxidation
Hydroperoxides
Aldehydes,
Ketones, Diene
and triene
Acids, Epoxides,
Dimers, Oxirane
rings
Outline of the
process

9. Photo-oxidation
Sunlight
Photosensitizers
Singlet oxygen
Natural Pigment
Chlorophyll, Pheophytin, Flavin,
Porphyrin
Dye Stuff
Erythrosine, Methylene blue
13
Triplet oxygen
Singlet oxygen

10. Why singlet
oxygen??

11. Angular momentum = 2s + 1
(s is total spin)
For Triplet:
2(1/2+1/2) + 1= 3
For Singlet:
2(1/2- 1/2) + 1 = 1

12. sens + RH(sing) + hv → intermidiates-I(trip)
intermediats-I(trip) + O2(trip) + hv → ROOH + sens
sens + O2((trip)) + hv → intermidiates-II(sing)
intermediats-II(sing) + RH(sing) + hv → ROOH + sens
Pathways have
been proposed
Pathway-I
Pathway-II
sens - photosensitizes

13. Mechanism
RH + O2(singlet) + hv → ROOH
Singlet oxygen is more electrophilic than triplet oxygen and can reacts 1500
times greater than triplet oxygen with high electron density moieties like, C=C
Hence it’s a type of ene type reaction,

14. Evidences show that it is a Free radical mechanism
Inhibition in rate by chemical species
known for interfere with Free radical
reaction.
Catalysed by light
High yield of
Hydroperoxides
Relatively long
Induction Period
Auto-Oxidation

15. Experimental results are explained in three step free radical scheme
RH
RH + O2 R’
R’ + O2 ROO’
ROO’ + RH ROOH +
R’ + R’
R’ + ROO’
ROO’ + ROO’
K1
K2
K3
K4
K5
K6
Initiation
Propgation
Termination
R’
Nonradical
product

16. Rate equations:
Oxygen Absorption:
Eqn (4), (5) can
be neglected
Eqn (5), (6) can
be neglected

17. Hydroperoxides and their DecompositionOxygen-oxygen
bond scission
Alkoxy and
Hydroxyl radicals
Aldehyde and acid/ester
Or
Hydrocarbon and oxoacid /
oxoester
1
5
4
3
2
Ketone and
1ᵒ Alcohol
2ᵒ Alcohol and Free
radical
Ketone and
Hydrocarbon

19. A thermodynamically difficult reaction because Activation energy is 35kcal/mol
Metal Catalysts
Prooxidants
This reaction is still a considerable mistery

20. Pro-oxidants
Transition metals, particularly those having two or more valence states with a
suitable oxidation-reduction potential between them are major pro-oxidants.
Initiate / Accelerate the reaction
Sources
1. Traces metals are naturally occurring components of all food tissues.
2. Originate from the soil where bearing material grow.
3. Metallic equipments used in handling, extraction and processing
4. Residual quantity of Spent bleaching earth, Nickel catalyst etc.
1
3
2

21. Factors Influencing the
rate of Lipid Oxidation
Fatty Acid Composition
Free Fatty Acid
Temperature
At lower content no effect on oxidation
but in higher, it promotes
Number, position and
geometry of double bonds
As temperature increase rate of
oxidation increase but simultaneously
availability of oxygen decrease
Pro-oxidants

22. Surface Area
Oxygen Concentration
Rate of oxidation is proportional to
the surface area exposed to air or
water(in case of emulsion)
Supply of oxygen is unlimited so rate is
independent of oxygen pressure, but at
very low pressure rate is proportional to
oxygen pressure
Effective promoter of lipid oxidation The Ea’s
for C-H and C-C scission reactions are
slightly higher than bond energies 98.4
kCal/mol and 83.1 kCal/mol, respectively,
and this is equal to photon energy available
only at wavelengths <254 nm (UV Region)
Radiant Energy

23. Synergists
Moisture
Metal chelating agent deactivate the metals
hence no further initiation takes place
Citric acid, Ascorbic acid, EDTA etc.
Water activity measurement is a new
concept, for dried food aw < 0.1 (No such
effect on rate of Oxidation is detected)
As increasing the aw and keeping
between 0.65 - 0.85 increase the rate
and beyond the range oxidation rate is
enormously high and constant to aw
Anti-oxidant

24. Anti-oxidants
Monohydric and Polyhydric phenols derivatives are excellent hydrogen or
electron donators and their radical are highly stable due to resonance
delocalization
Natural Anti-oxidants Synthetic Anti-oxidants
Sesamol
Gossypol
Tocopherol
Tocotrienol
Ginistein
Flavonoids
BHT
BHA
TBHQ
PG
TBHP
Antioxidants do not prevent oxidation but they slow it down, thereby extending
the induction period and hence the shelf-life of fat-containing foods.
Prevention

25. Mechanism
Photo-oxidation is not inhibited by the antioxidants used for autoxidation but by
singlet oxygen quenchers of which the best known is -carotene.

26. Synthetic
Antioxidants

27. Natural
Antioxidants
Gossypol

28. Natural
Antioxidants

29. 1. Peroxide Value
2. p-Anisidine Value
3. Ultra Violet Spectroscopy
4. Total Polar Component
5. Iodine Value
6. Active Oxygen Method
Measurement of
Oxidative Rancidity

30. 1. Peroxide Value
Indicator of initial oxidation suffered by sample.
Measured on the basic of ability of peroxides to liberates iodine from
potassium iodide
Expressed in terms of milliequlivents of oxygen per kilogram of fats.

31. 2. Anisidine Value
In the presence of acetic acid, p-anisidine react with aldehyde
producing a yellowish colour.
The molar absorbance at 350nm increases if the aldehyde
contains a double bond conjugated to the carbonyl double bond.

32. 3. Thiobarbituric Acid Test
Oxidation products of unsaturated system produce a colour reacton
with TBA.
Many alkanals and alkenals produces yellow colour compounds at
450nm
Dienal produce a red colour pigment at 530nm

33. General characteristics
curve of fat oxidation
Induction Period
Without antioxidant
Induction period:
After a certain critical amount of oxidation has occurred, the reaction
enters to its second phase characterized by a rapid accelerating rate of
oxidation, this period is called induction period

34. With antioxidant
Induction Period

35. General
Information
Level of Anti-oxidants
BHA NMT 100 ppm
PG NMT 200 ppm
BHT NMT 200 ppm
TBHQ NMT 200 ppm
Natural Anti-Oxidant GMP
Effect of Free radicals on cell

36. Quick History of Lipid
. Oxidation
Henri Braconnot, 1815
Acids were formed
during rancidification
J.L. Bolland,1947
Detailed kinetic study of
antioxidant (ethyl linoleat and
hydroquinone)
F.D. Gunstone, 1943-47
Auto-oxidation and
effect of temperature
T.P. Hilditch,1943-47

37. M. Karel, 1988
Combined study of lipid oxidation
and water activity(aw)
A. Tappel, 1950
Vitamin E as a biological
antioxidant
Gilmont and Levenson,
1946
Photooxidation
H.S. Rawals, 1970
Fat oxidation
characteristics and
induction period

38. BIBLIOGRAPHY